The present invention relates to CATV systems. More particularly, the invention relates to a remote line monitoring technique for a CATV system in which a center facility can collectively monitor the conditions of lines connected between the center falicity and terminal units.
In a CATV system, typically coaxial cables are employed to connect the center facility to the terminal units. In order to satisfactorily maintain the function of the coaxial cables, gas is continuously supplied in to the cables under a pressure slightly higher than the atmospheric pressure. It is essential to monitor the gas pressure so that it is maintained at a predetermined value at all times. In addition, it is necessary to monitor whether or not the signal levels and signal amplification factors at various points along the lines are correct and whether or not the supply voltage applied to the line amplifiers is satisfactory.
In a conventional monitoring technique, several monitoring points are provided on the lines between the center facility and the terminal units. Technicians must periodically visit the monitoring points to determine the line conditions. Accordingly, the detection of abnormalities on the lines takes much time and labor.
In order to maintain the signal level constant, a technique has previously been employed in which the signal level is monitored by the provision of a pilot signal of constant level outside of the communications frequency band used for normal signal transmission in the system. The amplification factor is maintained constant by measuring the pilot signal at points where main cable amplifiers are provided. In the case where the lines of the CATV system are branched, pilot signals transmitted from the branch lines towards the center facility are collectively applied at the branching points. Therefore, in practice, it is necessary to provide different pilot signals of different but close frequency from the various lines and to separate the various pilot signals for one another for measurement thereby to monitor the conditions of the branch lines. Therefore, the above-described conventional method is disadvantageous in that a device for separating and selecting the pilot signals of different frequency is necessarily intricate in construction and high in manufacturing cost. Moreover, the line condition monitoring must be carried out through approximation measurement because the pilot signals are out of the communications frequency band.